1. Field of the Invention
The present invention relates generally to a predistorter, and in particular, to a predistorter for linearizing the nonlinearity of a power amplifier in a system supporting a multimode and a multiband (frequency band).
2. Description of the Related Art
With the development of mobile communication, a radio frequency (RF) system aims at providing a multimode and a multiband. Further, the RF system requires a small size, high efficiency, and high linearity. However, an amplifier of a transmitter in a mobile communication system consumes a large amount of power and also has nonlinearity, which causes degradation of the entire system.
Accordingly, research is being conducted on linearization of an amplifier. Among the research results, predistortion is a representative technique that can be miniaturized enough to be used in a mobile terminal for mobile communication.
FIG. 1 illustrates basic principles of a predistorter. Referring to FIG. 1, AM-AM distortion of a power amplifier exhibits a gain that decreases as an input power Pin increases. In addition, AM-PM distortion exhibits a phase that increases or decreases as the input power Pin increases. Therefore, the predistorter and the power amplifier have contrary characteristics, which achieve overall linearity.
FIG. 2 is a circuit diagram of a conventional parallel diode predistorter. Referring to FIG. 2, the conventional parallel diode predistorter includes a diode D2 having a cathode connected to the ground, a bias feed resistance R2 connected between an anode of the diode D2 and a bias voltage VCC, a capacitor C21 connected between an input terminal IN and the anode of the diode D2, and a capacitor C22 connected between the anode of the diode D2 and an output terminal OUT.
In the conventional parallel diode predistorter, because a voltage Vd at the diode D2 decreases as an input power increases, the resistance of the diode D2 increases. That is, as the input power increases, the conductance of the diode D2 decreases and thus a bias point changes as illustrated in FIG. 3.
In FIG. 3, L indicates a bias point when the input power is large, and S indicates a bias point when the input power is small.
In the conventional parallel diode predistorter, the capacitors C21 and C22 are arranged in parallel with the diode D2 to control a phase deviation, thereby controlling a susceptance.
Once a conductance G and a susceptance B according to an input power are acquired, a gain and a phase of a predistorter can be calculated using Equations (1) through (3).
                              S          ⁢                                          ⁢          21                =                              2                          2              +                                                Z                  T                                ⁢                Y                                              =                                    2                              2                +                                                      Z                    T                                    ⁢                  Y                                                      =                          2                              2                +                                                      Z                    T                                    ⁡                                      (                                          G                      +                      jB                                        )                                                                                                          (        1        )                                                                S            ⁢                                                  ⁢            21                                    =                  2                                                                      (                                      2                    +                                                                  Z                        T                                            ⁢                      G                                                        )                                2                            +                                                (                                                            Z                      T                                        ⁢                    B                                    )                                2                                                                        (        2        )                                          ∠          ⁢                                          ⁢          S          ⁢                                          ⁢          21                =                              tan                          -              1                                ⁡                      (                                                            -                                      Z                    T                                                  ⁢                B                                            2                +                                                      Z                    T                                    ⁢                  G                                                      )                                              (        3        )            
More specifically, Equation (2) calculates a gain of the predistorter and Equation (3) calculates a phase of the predistorter.
According to Equations (1) through (3), as the conductance G decreases, the size of S21, i.e., the gain of the predistorter, increases and the phase of S21, i.e., the phase of the predistorter, decreases with an increase in the input power.
However, to implement a predistorter suitable for a multimode and a multiband, the predistorter should have characteristics as illustrated in FIGS. 4 and 5. That is, the predistorter should provide a gain change 41, a free change in turning points 42 and 43 at which a gain starts to change according to an input power, free turns 44 and 45 of a gain, a phase change 46, a free change in turning points 47 and 48 at which a phase starts to change according to an input power, and free turns 49 and 50 of a phase.
However, because the conventional parallel diode predistorter illustrated in FIG. 2 should control the bias voltage VCC or use a buffer amplifier or an attenuator to control a turning point, it consumes a large amount of power and cannot provide gain and phase characteristics suitable for multimode, multiband power amplifiers.